Radiant barriers are installed in homes, usually in attics, marketed primarily to reduce summer heat gain in warm climates and winter heat loss in cold climates, helping to keep energy costs low. Barriers are normally comprised of a highly reflective material, typically aluminum that reemits radiant heat from the sun back into the environment, rather than absorbing it. When the sun heats a roof, it is primarily the sun's radiant energy that makes the roof hot. A large portion of this heat travels by conduction through the roofing materials to the attic side of the roof. The hot roof material then radiates its gained heat energy onto the cooler attic surfaces, including the air ducts and the attic floor.
A radiant barrier reduces the radiant heat transfer from the underside of the roof to the other surfaces in the attic. A radiant barrier's performance is determined by three factors: emissivity, reflectivity, and the angle of radiation. Emissivity is the ratio of the radiant energy leaving a surface to that of a black body at the same temperature and with the same area. It is expressed as a number between 0 and 1, the higher the number, the greater the emitted radiation. The second factor is reflectivity, a measure of how much radiant heat is reflected by a material. It is also expressed as a number between 0 and 1, the higher the number, the greater the reflectivity. The third factor is the angle at which the incident radiation strikes the surface—a right angle (perpendicular) usually works best. All radiant barriers must have a low emissivity (0.1 or less) and high reflectivity (0.9 or more).
The current market for radiant barriers has two main functional types in the form of foil chips and foil rolls. Both types are made of the same materials, but are used in different applications within the housing industry. Studies have shown these products are successful, however, maintenance issues have proved to be an area of concern for both chips and rolls due to the accumulation of dust. Chips must be blown into attic spaces, increasing installation costs, and thus reducing their penetration into the radiant barrier market.
In recent years, the widespread use of radiant barriers has fallen out of favor in the building industry. Much of this is due to a lack of quantifiable results, and a technology that was rushed to market and not fully re-engineered or properly calibrated for building applications.